1. Field
The present embodiments relate to a spindle drive and a patient positioning system.
2. Related Art
Spindle drives and threaded spindles are used in applications in which, for example, loads or system components have to be moved and precisely positioned. Spindle drives have been used in industrial applications such as machine tools and end user applications such as garage door openers. In addition, spindle drives are used in, for example, medical diagnostic and/or therapeutic devices (DT devices). For example, X-ray tube assemblies, X-ray image receivers, patient positioning systems, computer tomography equipment, magnetic resonance equipment or nuclear medicine and radiation therapy equipment, are moved and positioned using spindle drives.
Spindle drives may experience unidentified wear or mechanical failure, which creates considerable risks of injury and endangerment for the user or for other persons entering into contact with the spindle-driven apparatus, for example, in X-ray devices for the doctor or the patient. Generally, the spindle nut revolves on a threaded spindle, with an epicyclic gearing, for example, a recirculating ball gear. Unidentified wear, which leads to complete mechanical failure of the spindle drive, may be caused by the failure of the recirculating ball gear. If the balls of the recirculating ball gear are destroyed by wear, for example, then the recirculating ball gear is no longer in mutual engagement with the threaded spindle. The connection between the spindle and the mass to be moved is lost. The recirculating ball gear is no longer supported by the balls in the thread of the threaded spindle. The recirculating ball gear becomes freely movable. In a vertical spindle drive, the mass to be moved, in the worst scenario, falls down.
In order to reduce the risk of danger, due to an unforeseeable failure of the spindle drive, recirculating ball gears with catch nuts (safety nuts) are used. The catch nut is fixedly connected to the spindle nut and has a thread that is in mutual engagement with the thread of the threaded spindle, which is similar to the balls of the recirculating ball gear. The pitch of the thread of the catch nut here corresponds to the pitch of the thread of the spindle nut and to that of the thread of the threaded spindle. The catch nut can revolve jointly with the recirculating ball gear around the threaded spindle. For example, the thread of the catch nut revolves frictionlessly around the threaded spindle when the spindle nut, or, for example, the recirculating ball gear revolves truly around the spindle (i.e. within the predefined tolerances).
The catch nut is engaged when the spindle nut is no longer revolving truly around the spindle. For example, the spindle nut may be damaged or no longer revolve truly around the spindle because of excessive bearing play of the recirculating ball gear or destruction of the recirculating balls. When the spindle nut is damaged or no longer revolves truly around the spindle, the force closure between threaded spindle and recirculating ball gear is lost. Alternatively, the force closure between the catch nut and threaded spindle is established. An uncontrolled self-movement of the mass to be moved is then prohibited by the catch nut. The mass to be moved is prevented, for example, from falling down.
An operation of the spindle drive despite failure of the spindle nut is possible because the thread of the catch nut substantially corresponds to that of the spindle nut. When the spindle nut experiences mechanical failure, the catch nut steps in for the spindle nut and replaces it in its function.
Generally, a spindle drive and catch nut are not visible to a user, but are hidden, for example, behind panels or cladding parts. Accordingly, the failure of the spindle drive is not apparent to the user. If the spindle nut fails and the operation is continued on the basis of the catch nut, no further safeguard exists. If the catch nut subsequently fails, there complete failure of the spindle drive, for example, a height-adjusting device falls down.
Generally, in spindle drives with a fixed spindle nut and rotating spindle, the mutual connection of a catch nut and a spindle nut is frequently checked, for example, by an electrical device. For example, when the spindle nut fails and falls onto the catch nut, a mutual contact is formed, which is electrically or mechanically detectable. This mutual contact is detected and is used to activate an emergency off switch. However, in spindle drives with a fixed spindle, the spindle nut is driven and revolves around the spindle. Accordingly, checking the contact between the spindle nut and the catch nut is not readily possible because, for example, electrical contact between the two rotating nuts would be necessary. To create a contact between the two rotating parts is complicated. Standard contact types, for example, sliding contacts, are subject to wear and thus prone to error.
GB 896 914 C describes an apparatus for indicating an admissible wearing of a first nut. The first nut bears a load and, because of wearing, is subject to displacement along a threaded spindle. The apparatus contains a second nut, which bears no load and in which said threaded spindle engages. Only a relative axial displacement is allowed between the two nuts. The second nut has an adjustable stopper, which actuates a switch on the first nut whenever the relative axial distance between the two nuts falls below a value corresponding to the admissible wearing of the thread of the first nut. The switch is connected to a display means, which is switched on when the switch is actuated.
A spindle drive with a fixed spindle and revolving spindle nut and catch nut that is able to identify the failure of a spindle nut is desired.